1. Field of the Invention
The present invention relates to a method for manufacturing micro-structural units such as micro-mirror elements, acceleration sensors, angular velocity sensors, transducer element and the like that are manufactured by micro-machining techniques.
2. Description of the Related Art
In recent years, elements having microscopic structures that are formed by micro-machining techniques have been applied in various technical fields. For example, extremely small micro-mirror elements that have a light reflecting function have attracted attention in the field of optical communications technology.
In optical communications, light signals are transmitted using optical fibers as a medium, and so-called optical switching devices are generally used for the switching of light signals from fibers that form the transmission paths to other fibers. Examples of characteristics that are required in an optical switching device in order to achieve favorable optical communications include high capacity, high speed, high reliability and the like in the switching operation. From the standpoint of such characteristics, there have been increased expectations for devices that incorporate micro-mirror elements manufactured by micro-machining techniques as optical switching devices. The reason for this is as follows: if micro-mirror elements are used, switching processing between the optical transmission path on the input side and the optical transmission path on the output side in the optical switching device can be performed without converting the light signals into electrical signals, so that such a system is suitable for obtaining the characteristics described above.
Micro-mirror elements comprise a mirror surface that is used to reflect light, and the elements can vary the direction of reflection of the light by swinging this mirror surface. Electrostatically driven micro-mirror elements which utilize an electrostatic force to swing the mirror surface are used in most devices. Electrostatically driven micro-mirror elements can be divided into two main categories, i.e., micro-mirror elements that are manufactured by so-called surface micro-machining techniques, and micro-mirror elements that are manufactured by so-called bulk micro-machining techniques.
In the case of surface micro-machining techniques, thin films of materials corresponding to the respective constituent parts are machined into desired patterns on the surface of a substrate, and respective parts that constitute the element, such as the supporting body, mirror surface, electrode parts and the like, as well as sacrifice layers that are later removed, are formed by successively laminating such patterns. On the other hand, in the case of bulk micro-machining techniques, the supporting body, mirror part and the like are formed into desired shapes by etching substrates of the materials themselves, and the mirror surface and electrodes are formed as thin films if necessary. For example, bulk micro-machining techniques are described in Japanese Patent Application Laid-Open No. H10-190007, Japanese Patent Application Laid-Open No. H10-270714 and Japanese Patent Application Laid-Open No. 2000-31502.
One technical feature that is required in micro-mirror elements is a high degree of flatness of the mirror surface that reflects the light. However, if a surface micro-machining technique is used, since the mirror surface that is ultimately formed is thin, this mirror surface tends to become easily curved. Accordingly, it is difficult to achieve a high degree of flatness in a mirror surface that has a broad area. On the other hand, if a bulk micro-machining technique is used, the mirror part is formed by etching a relatively thick material substrate itself by means of an etching technique, and a mirror surface is disposed on top of this mirror part. Accordingly, even in the case of a mirror surface with a broad area, the rigidity of this mirror surface can be ensured. Consequently, a mirror surface that has a sufficiently high degree of optical flatness can be formed.
FIG. 32 is a partially cut-away perspective view of a conventional micro-mirror element X5 mad by a bulk micro-machining technique. This micro-mirror element X5 has a mirror part 510 which has a mirror surface (not shown in the figures) formed on the upper surface, an inner frame 520 and an outer frame 530 (partially omitted), and comb tooth-shaped electrodes are integrally formed on each of these parts. In the mirror part 510, comb tooth-shaped electrodes 511 and 512 are formed on a pair of the end portions. In the inner frame 520, comb tooth-shaped electrodes 521 and 522 that extend inward are formed facing the comb tooth-shaped electrodes 511 and 512, and comb tooth-shaped electrodes 523 and 524 that face outward are also formed. In the outer frame 530, comb tooth-shaped electrodes 531 and 532 that extend inward are formed facing the comb tooth-shaped electrodes 523 and 524. Furthermore, the mirror part 510 and inner frame 520 are connected by a pair of torsion bars 540, and the inner frame 520 and outer frame 530 are connected by a pair of torsion bars 550. The pair of torsion bars 540 define the axis of rotation A5 of the rotating action of mirror part 510 with respect to the inner frame 520, and the pair of torsion bars 550 define the axis of rotation A5′ of the rotating action of the inner frame 520 with respect to the outer frame 530, and the accompanying rotating action of the mirror part 510.
In the micro-mirror element X5 constructed in this manner, the set of comb tooth-shaped electrodes that are disposed in close proximity in order to generate an electrostatic force, e.g., the comb tooth-shaped electrodes 511 and 521, adopt an orientation in which the electrodes are separated into two tiers above and below as shown in FIG. 33A when no voltage is applied. On the other hand, when a specified voltage is applied, the comb tooth-shaped electrode 511 is drawn into the comb tooth-shaped electrode 521 as shown in FIG. 33B; as a result, the mirror part 510 swings. More specifically, when the comb tooth-shaped electrode 511 is positively charged and the comb tooth-shaped electrode 521 is negatively charged, for example, the mirror part 510 rotates about the axis of rotation A5 while twisting the pair of torsion bars 540. On the other hand, when the comb tooth-shaped electrode 523 is positively charged and the comb tooth-shaped electrode 531 is negatively charged, the inner frame 520 and accompanying mirror part 510 rotate about the axis of rotation A5′ while twisting the pair of torsion bars 550. Such rotational driving of the mirror part 510 allows the switching of the direction of reflection of the light that is reflected by the mirror surface (not shown in the figures) disposed on the mirror part 510.
FIG. 34 shows a conventional method used to manufacture the micro-mirror element X5. In FIG. 34, the formation processes of a portion of the mirror part 510, the inner frame 520, the outer frame 530, the torsion bars 540 and portions of the set of comb tooth-shaped electrodes 511 and 521 shown in FIG. 32 are expressed by changes in a single cross section. This single cross section is expressed as a continuous cross section by modeling a plurality of cross sections contained in a single micro-switching element formation region in the material substrate (wafer) on which machining is performed.
In the manufacturing method of the micro-mirror element X5, a wafer S5 such as that shown in FIG. 34A is first prepared. The wafer S5 is a so-called SOI (silicon on insulator) wafer, and has a laminated structure comprising a silicon layer 501, a silicon layer 502 and an insulating layer 503 located between these silicon layers. Next, as shown in FIG. 34B, anisotropic etching is performed on the silicon layer 501 via a specified mask so that the structural parts that are to be formed in the silicon layer 501, such as the mirror part 510, a portion of the inner frame 520, a portion of the outer frame 530, the torsion bars 540, the comb tooth-shaped electrode 511 and the like, are formed.
Next, as shown in FIG. 34C, anisotropic etching is performed on the silicon layer 502 via a specified mask so that the structural parts that are to be formed in the silicon layer 502, such as a portion of the inner frame 520, a portion of the outer frame 530, the comb tooth-shaped electrode 521 and the like, are formed. Next, as shown in FIG. 34D, isotropic etching is performed on the insulating layer 502 so that locations that are exposed in the insulating layer 503 are removed. In this way, the mirror part 510, inner frame 520, outer frame 530, torsion bars 540 and set of comb tooth-shaped electrodes 511 and 512 are formed. The other sets of comb tooth-shaped electrodes are formed in the same manner as the comb tooth-shaped electrodes 511 and 521, and the torsion bars 550 are formed in the same manner as the torsion bars 540.
In the micro-mirror element X5, since the respective comb tooth-shaped electrodes are displaced along with the rotating action of the mirror part 510 and inner frame 520, the respective comb tooth-shaped electrodes must have a sufficient thickness that is suited to the inclination angle of the mirror part 510 and inner frame 520. This thickness corresponds to the dimension of the wafer S5 in the direction of thickness. For example, in a case where the length D of the body part 513 of the mirror part 510 is 1 mm, if the mirror part 510 is inclined by 5° about the axis of rotation A5 with respect to the inner frame 520, then one body end part 513′ will sink in by 44 μm. Accordingly, the thickness of the comb tooth-shaped electrodes 511 and 512 of the mirror part 510 must be 44 μm or greater.
From the standpoint of obtaining a large inclination angle using a small applied voltage, it is desirable to form the torsion bars 540 and 550 (which have a resistance to twisting) with a small thickness in order to reduce this resistance to twisting. However, in the case of the conventional micro-mirror element X5 described above, the torsion bars 540 and 550 are formed with the same thickness as the respective comb tooth-shaped electrodes, and are therefore thick. For example, if the thickness of the comb tooth-shaped electrodes 511 and 512 is set at 44 μm or greater as described above, then the thickness of the torsion bars 540 and 550 is also 44 μm or greater. In the micro-mirror element X5, since the torsion bars 540 and 550 are thus thick, the electrostatic force that must be generated between the comb tooth-shaped electrodes in order to twist these torsion bars is relatively large; accordingly, the required driving voltage is relatively large.
Furthermore, from the standpoint of reducing the weight of the mirror part 510 and thus lessening the inertia of this part, it is desirable to form the mirror part 510 with a small thickness. However, in the conventional micro-mirror element X5 described above, the mirror part 510 as a whole is formed (for example) with the same thickness as the comb tooth-shaped electrodes 511, 512, 523 and 524, and is thus thick. Since the mirror part 510 as a whole is thus thick, the mass of the mirror part 510, and therefore the inertia of this part, is relatively large. As a result, in the micro-mirror element X5, there may be cases in which the desired frequency response characteristics cannot be obtained with regard to the rotational displacement of the mirror part 510. Furthermore, in order to increase the rotational angle in the rotational displacement of the mirror part 510, it is necessary to increase the thickness of the comb tooth-shaped electrodes and increase the driving force. However, in the micro-mirror element X5, this increase in the thickness of the comb tooth-shaped electrodes leads to an increase in the mass of the mirror part 510, and thus to an increase in the inertia of the mirror part 510. Thus, in the micro-mirror element X5, there is some difficulty involved in obtaining good frequency response characteristics while achieving a large rotational angle of the mirror part 510.
Meanwhile, in the micro-mirror element manufacturing process described above, it is desirable from the standpoint of preventing damage to the wafer S5 that this wafer S5 be thick. However, in the above-described manufacturing method of the micro-mirror element X5, since the thickness of the wafer S5 is directly reflected in the total thickness of one set of comb tooth-shaped electrodes (e.g., the set of comb tooth-shaped electrodes 511 and 521), a wafer S5 with a thickness that is substantially the same as the total thickness of one set of comb tooth-shaped electrodes in the micro-mirror element X5 that is the object of manufacture must be used. For example, in a case where the total thickness of one set of comb tooth-shaped electrodes that is to be formed is 200 μm, a wafer S5 with a thickness of 200 μm must be used in order to form a micro-mirror element X5 that has such a pair of comb tooth-shaped electrodes. In cases where the total thickness of the wafer S5 is less than about 200 μm, damage to the wafer S5 tends to occur in the element manufacturing process; accordingly, the mass production of such elements is difficult.
In micro-structural units that are manufactured by micro-machining techniques, the various structural parts that are formed by etching in the material substrate (wafer) have respective desired thicknesses in most cases (as was described above). However, in conventional bulk micro-machining techniques, since the degree of freedom regarding the thickness dimensions of the respective structural parts in a single micro-structural unit is low as was described above, it tends to be difficult to realize the desired thickness dimensions for each of a plurality of structural parts that have different thicknesses.